The present invention relates to the protection of a ferrocarbon substrate with a corrosion resistant coating for use as a cell casing or other component in a sodium-sulfur cell.
The cell casing of a sodium-sulfur cell, especially the casing for the sulfur compartment, is subject to severe corrosive attack. Relatively inexpensive carbon steels cannot be employed as the casing for the sulfur compartment. They form metal sulfide scales which lead to extensive physical degradation and contamination of the sulfur/sodium polysulfide melt under cycling. This interferes with efficient cell operation and causes cell capacity losses, cell resistance increases and degradation of the electrolyte.
Breiter U.S. Pat. No. 3,959,013 discloses the use of a sulfur casing portion of metal, including steel, by providing a corrosion resistant and electronically conducting layer to adhere to its inner surface. The disclosed layer is formed of molybdenum or graphite by plasma spraying. In U.S. Pat. No. 4,048,390, an iron alloy is coated with aluminum by pack aluminiding.
Corrosion protective layers have been formed on substrate metals for a variety of uses other than the corrosive environment of a component for a sodium-sulfur cell. For example, carbon steel containers have been chromized by a variety of processes including chromium diffusion by pack chromizing for applications such as high temperature turbine blades. One process known as the D.A.L. process, is described in Samuel, R. L. and Lockington, N. A., Metal Treatm. 18 (1951) 407, 440, 495. In this process, ammonium iodide is packed with ferrochromium and kaolin powder to form a chromium-carbon-iron alloy layer. Other procedures such as the BDS process have also been employed in a diffusion process for forming a chromium alloy. There, a mixture of ferrochromium and broken pieces of ceramic are heated in a retort with circulating hydrogen chloride gas. The hydrogen chloride reacts with the ferro-chromium to form chromous chloride which is absorbed in the ceramic. Thereafter, the chromous chloride is volatilized and chromizes the article by replacement and reduction reactions. This procedure is described in A. H. Sulley, Chromium, Butterworth Scientific Publication, London, (1974), 197-199.